The present invention is related to a method of filtering molten aluminum and aluminum alloys. More specifically the present invention is related to a method for filtering molten aluminum and aluminum alloys comprising maintaining a partial pressure of oxygen within the interstitial spaces of the filter to mitigate reactivity between the oxides making up the ceramic filter and aluminum or aluminum alloys.
Filtration of molten metals is a common practice in industry. In particular, filtration of molten aluminum using reticulated foam filters is widely practiced in the art. It is well known that the refractory and chemical stability of the filter material is of critical importance in obtaining effective inclusion particle retention and to avoid release of reaction products from the filter material into the molten metal as a contaminant.
Reticulated foam filters using alumina aggregate grain in a binder phase have been commonly used for filtering aluminum and aluminum alloys. Aluminum orthophosphate has been widely practiced as exemplified in U.S. Pat. Nos. 3,947,363; 3,962,081 and 4,343,704. Unfortunately, it is widely realized in the art that aluminum phosphate has poor resistance to chemical attack, particularly, when used with magnesium containing aluminum alloys. During use the magnesium corrodes the intergranular areas causing a softening and release of alumina grain as inclusion material. Softening is known to compromise the strength of the reticulated foam structure. Yet another problem with aluminum orthophosphate based filters is the potential environmental issues resulting from formation of phosphine gas from the spent filter.
High purity sinter bonded alumina filters using an alumina hydrate bonding system are also well known as exemplified in U.S. Pat. No. 4,610,832. These filters exhibit excellent chemical resistance to attack from magnesium, but the expensive reactive grade alumina coupled with the high sintering temperatures eliminates these filters from being financially feasible for use in many processes.
In addition to the aforementioned issues, alumina filters, both phosphate bonded and sinter bonded, suffer from poor thermal properties due to their high thermal expansion and compressive modulus.
It would be desirable to use silica as a refractory foam material, as it tends to be inexpensive, easily processed. Silica also has a low thermal expansion and compressive modulus. Unfortunately, it is known to be highly reactive in molten aluminum alloys, especially in magnesium containing alloys.
An Ellingham free-energy diagram is illustrated in FIG. 1 wherein provided is the free energy of oxide formation as a function of temperature. Aluminum and magnesium are well below that of silicon. Therefore, the presence of unprotected silica in either the grain material or binder phase of a reticulated foam filter would be readily attacked by aluminum or magnesium. It is widely understood in the art that silica is to be avoided for use when filtering molten aluminum, magnesium or alloys thereof. Exclusion of silica is most unfortunate due to the low raw material cost and the improved thermal and mechanical properties. Silica containing materials have relatively low thermal expansion and compressive modulus. Silica is widely available in materials including mullite, kyanite, fused silica, calcium silicate, magnesium silicate, cordierite, and lithium aluminum silicates.
Boron glass can be used to improve the corrosion resistance of filter materials containing silica-rich aggregate grain materials. Under standard conditions of use in typical industrial aluminum filtration operations there is still some degree of attack, particularly, with magnesium-alloy containing materials.
There has been a long standing desire for a refractory material which is relatively inexpensive, robust under filtering conditions, non-reactive and which can be used for a wide variety of aluminum alloys including, especially, those containing magnesium. The present invention provides a method for filtering molten aluminum and alloys thereof which meets the long-standing desire in the art.